A linear image sensor is usually constituted by a single column of photo sensors such as photodiodes and it carries out only transverse scanning while vertical scanning is carried out mechanically or optically. The linear image sensor has been generally used for reading in a facsimile machine or an optical character recorder (OCR).
FIG. 14 is a diagram illustrating the construction of a prior art linear image sensor disclosed in the Japanese Patent Publication No.64-61163. In FIG. 14, reference characters R, G, and B designate photodiodes capable of receiving red signal light, green signal light, and blue signal light, respectively. A photodiode array 4 for detecting red signal light (hereinafter referred to as R) is constituted by a plurality of R photodiodes aligned in main scanning direction 10b. As well, a photodiode array 5 for detecting green light (hereinafter referred to as G) and a photodiode array 6 for detecting blue light (hereinafter referred to as B) are constituted by a plurality of G photodiodes and a plurality of B photodiodes, respectively, arranged in main scanning direction 10b. A B transfer part 3 successively reads out charges generated by the B photodiode array 6, a G transfer part 2 successively reads out charges generated by the G photodiode array 5, and an R transfer part 1 successively reads out charges generated by the R photodiode array 4. The R transfer part 1, the R photodiode array 4, the G transfer part 2, the G photodiode array 5, the B photodiode array 6, and the B transfer part 3 are arranged parallel to each other and perpendicular to a sub-scanning direction 10a. The image sensor has three pairs of a photodiode array and a transfer part adjacent thereto for R, G, and B. Herein, the B photodiode array 6 and the G photodiode array 5 are arranged opposite each other. Output terminals 40a, 40b, and 40c output signals transferred successively by the B transfer part 3, the G transfer part 2, and the R transfer part 1, respectively.
FIG. 15 is an enlarged view of the G transfer part 2. The G transfer part 2 comprises five stages of line memory (ME1 to ME5) 7, a horizontal transfer part 8 provided next to the last stage ME 5 of line memory 7, and an output terminal 40b of the horizontal transfer part 8, connected to an output amplifier 9.
Next, the operation of the prior art linear image sensor will be described with reference to green light (G). Signal charges which are generated in response to incident green light at each photodiode G of the G photodiode array 5 ar transferred into the G transfer part 2. Thereafter, these charges are transferred into the horizontal transfer part 8 through the line memory 7 which constitutes the G transfer part 2, and they are read out in series to the output amplifier 9.
In this prior art device, since the image sensor moves in the sub-scanning direction 10a to read the copy, the differences in reading positions between B and G and between B and R (clearances L.sub.1 and L.sub.2 in FIG. 14) should be compensated with time. For this reason, line memories 7 having a number of stages corresponding to the clearances L.sub.1 and L.sub.2 are provided in the G transfer part 2 and the R transfer part 1, respectively, whereby signal outputs obtained by the reading the same portion of the copy by the respective R, G, and B sensors can be output from the output amplifier 9.
In the above-described prior art image sensor, memories are required to compensate for the clearances between the respective photodiode arrays and in a case where the line memories 7 are built in the chip as described above, spaces accommodating the desired number of stages of line memories 7 are needed. Therefore, the chip width is increased and the number of chips that can be manufactured on a wafer is limited, thereby raising production cost. In addition an lengthy clock signal is required for operating the line memory 7.